To meet the future applications, it is needed to develop a reliable, accurate, miniaturized, low cost, kinematic global positioning system/inertial measurement unit integrated navigation system which is capable of operating in a high dynamic flight environment against a mixture of multi-type global positioning system (GPS) signal loss or deterioration, and improving navigation accuracy of GPS guided vehicle.
A major way of reducing cost of a navigation system is to use cheaper sensors and components that make the integrated navigation, and guidance and control system designs all the more challenging. Traditionally, guidance and navigation systems used for guided vehicle are mainly inertial navigation systems (INS) which is composed of an inertial measurement unit (IMU) and a processor. An important advantage of INS guidance is independence from external support. Unlike other types of guidance, INS devices can not be jammed or fooled by deceptive countermeasures. Unfortunately, INS guidance cannot provide high accuracy at long ranges. Inertial sensors are subject to errors that tend to accumulate over time--the longer the flight time, the greater the inaccuracy. The cost of developing and manufacturing a gyroscope increases as its level of accuracy improves. High-quality gyroscopes are difficult to manufacture, and only a relatively small number of companies around the world are capable of producing them. In part, this reflects the limited market for gyroscopes suitable for use in a highly accurate INS. Therefore, the inherent inaccuracy of the INS means that it cannot be the sole guidance system for a highly accurate tactical missile. Additional inputs are needed to correct for INS errors.
More recent developments in satellite navigation techniques are making possible the precise navigation at low cost. Efforts are now under way to develop integrated GPS/IMU navigation and guidance systems, suitable for use in high-jamming and high-dynamic flight environments. One implication of integrated GPS/IMU packages is that lower-cost, more easily manufactured IMU sensors can be used. This can result in significant savings.
Therefore, the technology trends for inertial sensors, GPS accuracies, and integrated GPS/IMU systems, including considerations of jamming and high dynamic, will lead to the one meter accuracy. The technical challenges come from the improvement of inertial and GPS sensor performance and the optimal integration of these sensors in the software and hardware designs. For inertial sensors, trend-setting sensor technologies are fiber-optic gyros, silicon micromechanical gyros, resonating beam accelerometers, and silicon micromechanical accelerometers. The utilization of these techniques is resulting in low-cost, high reliability, small size, and light weight for inertial sensors and for the systems into which they are integrated.
For the GPS accuracies, the current 16-meter (SEP, spherical error probable) specified accuracy, or 8 to 10-meter (CEP, circular error probable) observed accuracy of the GPS PPS (Precise Positioning Service) provides impressive navigation performance especially when multiple GPS measurements are combined into a robust centralized Kalman filter to update an INS . The filter provides an opportunity dynamically to calibrate the GPS errors, as well as, the inertial errors, and when properly implemented, CEPs far better than 8 meters can be obtained. For example, for precision guidance and automated aircraft landings, the requirement for accuracy of the integrated navigation systems is less than 3 meters or even better.
The trend towards improvement of accuracies of the integrated navigation systems is to utilize kinematic GPS and develop advanced fully-coupled kinematic GPS/IMU integrated systems in which both GPS receiver code and carrier tracking loops are aided with the inertial sensor information. Therefore, the measurement accuracy and anti-jamming capability of the GPS receiver can be dramatically enhanced and increased. Rapid carrier integer cycle ambiguity search and resolution, cycle slip detection and isolation procedures can also be completed within a few seconds through use of the inertial aiding information. In addition, GPS specified and observed current accuracies can be improved due to various stages of the wide area GPS enhancements.
As a result, the design and development of kinematic GPS/IMU integrated navigation systems is extremely challenging. Specifically, the hardware sensors and software algorithms constituting the system should satisfy the following requirements:
1. Inertial Sensors
Major changes are currently underway in technologies associated with inertial sensors used for stabilization, control, and navigation. These changes are enabling the proliferation of inertial sensors into a wide variety of new military and commercial applications. Main Challenges for design and fabrication of inertial sensors are low cost, high reliability, accuracy required by mission, small size, and lightweight.
(1) Fiber-Optic Gyros (FOG)
An economical replacement for the ring laser gyro (RLG) providing the same level of gyro bias performance. PA1 Continuous reduction in the gyro drift rate for more demanding applications.
(2) Silicon Micromechnical Gyros
(3) Resonating Beam Accelerometers
(4) Silicon Micromechanical Accelerometers
2. GPS receiver
As regards GPS sensor size, the current GPS receiving card (OEM, original equipment manufacture) is less than the size of a cigarette box. Technical trades for design of GPS receivers for GPS guided vehicle will focus on enhancement of high anti-jamming and high dynamic performance, and decrease of GPS measurement noise, including multipath effects.
(1) Trade-off between tracking loop bandwidth and high anti-interference of GPS receiver.
(2) Short time-to-first-fix (TTFF) and signal reacquisition time.
(3) Direct rapid P-code tracking and capturing.
(4) Inertial aiding code and carrier tracking loops.
(5) Receiver hardware/software digital signal processing.
(6) Anti-multipath antenna design.
3. Integrated System Algorithms
In future GPS/IMU integrated navigation systems, the fully-coupled integration requires that the GPS measurements and inertial sensor information are directly fused into a centralized navigation Kalman filter, and outputs of the filter can also aid the receiver tracking loops to improve the anti-jamming capability of the GPS receiver. Therefore, the technical challenges will be the following:
(1) System reconfiguration based on multiple sensors.
(2) Multi-mode robust navigation Kalman filter.
(3) Sensor failure detection and isolation.
(4) Inertial aiding on-the-fly phase ambiguity resolution and cycle slip detection.
(5) Rapid transfer alignment.
The current technical innovation will contribute significantly to the prospects for high dynamic vehicle proliferation. Historically, the most significant obstacles to the design and development of high dynamic guided vehicle have been the cost and complexity of vehicle guidance systems.